It's not because the discs are in their own individual cases, it's because they aren't sub-packaged in to individual series
They are in individual cases because they aren't sub-packaged in to the individual series (a symptom, not the cause), but they could also have been put in to one large digipak (or similar) and I believe that they still wouldn't be a "Multi Season" exception.
Had each series been placed in an individual digipak (or similar, say two discs per THINpak in this scenario), and then all of these been placed in one Slip Case, then that would be a "Multi Season", and could be dealt with as a Box Set like films. That makes sense, because each individual season is a physical entity inside the Slip Case, and can be uniquely identified as such.
But if that series sub-packaging doesn't have a unqiue UPC, then it does raise the question of how you would identify it: if it has multiple discs, you can't use Disc IDs
Grand Parent -> Series Collection -> Slip Case -> UPC
Parent -> Series 1 -> Digipak -> ???
Child -> Episodes -> Disc 1 -> Disc ID
Child -> Episodes -> Disc 2 -> Disc ID
Parent -> Series 2 -> Digipak -> ???
Child -> Episodes -> Disc 1 -> Disc ID
Child -> Episodes -> Disc 2 -> Disc ID
etc.
To me, in that situation, it would collapse back to a Parent->Child relationship
Parent -> Series Collection -> Slip Case -> UPC
Child -> Episodes -> Disc 1.1 -> Disc ID
Child -> Episodes -> Disc 1.2 -> Disc ID
Child -> Episodes -> Disc 2.1 -> Disc ID
Child -> Episodes -> Disc 2.2 -> Disc ID
which doesn't class it as "Multi Season"
But that is purely on a technical reason of not being able to properly identify the individual series packaging: if that were solved, then a "Multi Season" Box Set it could be
